(1) Field of the Invention
This invention relates to a semiconductor memory and a refresh cycle control method, and more particularly to a semiconductor memory, such as a dynamic random access memory (DRAM), which needs refresh operation and a refresh cycle control method.
(2) Description of the Related Art
A technique for making the oscillation cycle of a self-oscillator on the basis of which the refresh cycle of a DRAM is determined constant by using an electric current generated by a constant current generation circuit is known.
In a process of testing a device, a predetermined number of fuses from a plurality of fuses used in a circuit included in a semiconductor memory are programmed. By doing so, the level of an electric current generated by a constant current generation circuit can be changed and a refresh cycle can be set. Usually a refresh cycle becomes short if an electric current outputted from a constant current generation circuit is large. Conversely, if an electric current outputted from the constant current generation circuit is small, then the refresh cycle becomes long. The following descriptions are based on this premise.
Conventionally, a technique for changing the refresh cycle of a semiconductor memory including a temperature sensor according to temperature is known as a method for decreasing a standby current (see, for example, Japanese Unexamined Patent Publication Nos. 2003-5861 and 2003-100074).
FIG. 8 shows an example of changing a refresh cycle according to temperature.
In FIG. 8, data retention time (tREF) of a memory cell is also shown. In general, as temperature falls, data retention time of a memory cell included in a DRAM becomes longer. Data retention time of a memory cell included in a DRAM becomes approximately constant at temperatures lower than or equal to some temperature (about 20° C.). Data retention time of a memory cell included in a DRAM has the above temperature characteristic. Therefore, as shown in FIG. 8, a standby current can be reduced by shortening the refresh cycle (REF cycle) at temperatures higher than, for example, 60° C. and by lengthening the refresh cycle (REF cycle) at temperatures lower than or equal to 60° C.
As stated above, a refresh cycle can be changed by the level of an electric current outputted from a constant current generation circuit. However, this electric current depends on temperature. The dependence of this electric current on temperature may change according to parameters, such as the threshold voltage of a transistor included in the constant current generation circuit.
FIGS. 9A, 9B, and 9C show the temperature characteristic of an electric current generated by a constant current generation circuit and the temperature characteristic of a refresh cycle.
As shown in FIG. 9A, current Iref generated by the constant current generation circuit may increase or decrease with a rise in temperature. This depends on, for example, variation in the parameters of a transistor included in a constant current generation circuit in each chip. Hereinafter, the case where the current Iref increases with a rise in temperature will be referred to as positive temperature dependence and the case where the current Iref decreases with a rise in temperature will be referred to as negative temperature dependence.
As shown in FIG. 9B, if the dependence of the current Iref on temperature is negative, the slope of the refresh cycle in respect to temperature becomes negative accordingly. As shown in FIG. 9C, if the dependence of the current Iref on temperature is positive, the slope of the refresh cycle in respect to temperature becomes positive accordingly. In each of FIGS. 9B and 9C, the desired value (target) of the refresh cycle at temperatures lower than or equal to a cycle change temperature (60° C. in FIGS. 9B and 9C) at which the refresh cycle is changed and the desired value (target) of the refresh cycle at temperatures higher than the cycle change temperature are shown.
In a conventional process for testing a device, a refresh cycle has been adjusted by changing the level of the current Iref so that it will match the desired value of the refresh cycle at temperatures higher than the cycle change temperature at some temperature. In each of FIGS. 9B and 9C, the refresh cycle matches the desired value at a temperature of, for example, 95° C.
Conventionally, however, a refresh cycle extension rate at a cycle change temperature is uniform. If a refresh cycle is adjusted only on the basis of the desired value of the refresh cycle at temperatures higher than the cycle change temperature as shown in FIGS. 9B and 9C, then the refresh cycle will vary at room and low temperatures due to variation in the dependence of the current Iref on temperature in each chip. If the refresh cycle varies and becomes shorter than the desired value, then a standby current increases.
In addition, the following problem arises because a refresh cycle extension rate is uniform.
FIGS. 10A and 10B show how a conventional refresh cycle adjustment is made.
FIG. 10A shows how a conventional refresh cycle adjustment is made in the case of the dependence of the current Iref on temperature in a chip being negative. FIG. 10B shows how a conventional refresh cycle adjustment is made in the case of the dependence of the current Iref on temperature in a chip being positive. Data retention time (tREF) of a memory cell is also shown.
In the left-hand figure of FIG. 10A, for example, there is only a small difference between a refresh cycle and data retention time at a temperature near a cycle change temperature. If the refresh cycle exceeds the data retention time, data held in a memory cell is lost. Accordingly, it is desirable that there should be a large difference between the refresh cycle and the data retention time. As shown in the right-hand figure of FIG. 10A, by decreasing a refresh cycle extension rate at the cycle change temperature, the difference between the refresh cycle and the data retention time can be widened.
However, there is a case where the dependence of the current Iref on temperature in a chip is negative and where, as shown in FIG. 10B, a refresh cycle is shorter than a desired value at temperatures lower than or equal to a cycle change temperature. In such a case, the refresh cycle becomes still shorter than the desired value by decreasing a refresh cycle extension rate as with a chip in which the dependence of the current Iref on temperature is positive. As a result, a standby current increases further.